During this
treatment, the substance, at first very soluble, gradually loses its
solubility, and, when ready for analysis, has become wholly insoluble.
It is impossible at present to say whether it may not have undergone
other change; this is a matter as to which I hope to speak more
positively later. It is to be remarked, however, that these allotropic
forms of silver acquire and lose solubility from very slight causes,
as an instance of which may be mentioned the ease with which the
insoluble form B recovers its solubility under the influence of sodium
sulphate and borate, and other salts, as described in the previous
part of this paper.
The two insoluble forms of allotropic silver which I have described as
B and C--B, bluish green; C, rich golden color--show the following
curious reaction. A film of B, spread on glass and heated in a water
stove to 100 deg. C. for a few minutes becomes superficially bright
yellow. A similar film of the gold colored substance, C, treated in
the same way, acquires a blue bloom. In both cases it is the surface
only that changes.
_Sensitiveness to Light._--All these forms of silver are acted upon by
light. A and B acquire a brownish tinge by some hours' exposure to
sunlight. With C the case is quite different, the color changes from
that of red gold to that of pure yellow gold. The experiment is an
interesting one.
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